From under the coach I could see the access plate on the top of the tank. I made a mark on the side of the tank inline with the center of the plate. I then drove a nail up through the floor at the front of the tank. Measured from my mark on the side of the tank to the front and took the measurement inside and went from the nail back and that was where I made my first I inch hole to make sure I was over the center of the plate. My tank is located from side to side, directly in the center of the coach and the plate is in the center of the tank. I made a 6 inch sq hole, but if I were to do it again I would make it 7 inches. 6 was a little snug.